FOODS IN PLANTS 



39 



some of the compounds of nitrogen, potassium, phosphorus, or 

 other substances, these things may be made into more complex 

 foods known as proteins. 

 Many stages of the process 

 of making these more com- 

 plex foods are not known, 

 and these are too intricate for ex- 

 tended discussion at this time. The 

 leaf or other parts of the plant may 

 be used as the place where proteins 

 are made. They may be made im- 

 mediately after the carbohydrates, 

 or later, but sooner or later some 

 protein food is as necessary to the 

 continued life of plants as of animals. 

 The soil is the usual source of the 

 nitrogen, potassium, phosphorus, 

 and other substances that are used 

 in addition to the carbohydrates in 

 making proteins. Although the air 

 is 78 per cent nitrogen, this atmos- 

 pheric nitrogen is not available to 

 plants, except to certain bacteria 

 which are to be discussed later. 

 From the soil, compounds contain- 

 ing nitrogen and other substances 

 may be dissolved in water and then 

 carried into the plants. Fertile soils 

 are those which contain in available 

 form large quantities of the things 

 which plants use for food-making. 

 Replenishment and growth of new 

 parts can take place only by means 

 of foods, and since the plant makes 

 its own supply, the importance of 

 the process is very great. 



FIG. 27. Apparatus for collect- 

 ing oxygen from working plants 



Water plants are submerged with 

 one end in the mouth of the grad- 

 uate. Bubbles of oxygen pass 

 upward from the cut ends of the 

 stems and crowd out some water 

 from the previously filled grad- 

 uate. The ordinary test for oxy- 

 gen with a burning stick will 

 determine whether it is present. 

 In such an experiment care must 

 be taken to see that there is 

 plenty of space about the collect- 

 ing tube, to permit free passage 

 of the gases that are in the water. 

 After Ganong 



